Circuit packs and modules typically have one or more printed circuit board (PCB)-mounted integrated circuits (ICs) that dissipate enough heat that cooling by simple, un-enhanced natural convection, radiation and/or by heat conduction through the PCB is insufficient to keep junction temperatures below maximum operating limits. Generally, cooling of these ICs may be obtained by thermally connecting them to heat-dissipating structures, such as heat sinks, which in turn may be cooled by forced air when necessary.
Cooling using this general technique, however, is not always easy to achieve. For instance, variations in IC stack-up height and parallelism to the PCB present notable problems. For example, circuit packs utilized in high-speed optoelectronic and wireless communications applications have high-power components that must have EMI shielding. Given the EMI shielding requirements, these types of components are typically mounted on a PCB enclosed within a sealed aluminum box (i.e., the EMI shield) and mounted on racks within a certain product configuration (for example, the MetroEON™ product, commercially available from Lucent Technologies Inc. of Murray Hill, N.J.). However, air circulated by cooling fans in such products cannot penetrate the EMI shield thereby leading to disastrous results. For example, due to the high thermal resistance between such components their ambient component temperatures can be too high to achieve the desired product and individual component reliability and performance. Further, and potentially more damaging, elevated temperatures can destroy the components at certain higher temperature levels.
One way to combat the aforementioned heat build up is to provide a low thermal resistance path between the components (e.g., high-power ICs) within the EMI shield and a heat sink structure (e.g., a heat-spreader plate or cooling fins) external to the EMI shield. However, a problem that may be encountered in making a proper thermal connection between the ICs and the heat sink is that the distance between the heat sink and the ICs can vary, both because of IC stack-up height variations and because of thermal expansion of the entire assembly. As such, it is often difficult to achieve a proper, reliable contact between surfaces to maintain a good thermal path. Additionally, the two surfaces to be thermally connected may not be sufficiently parallel and in fact may shift relative to one another as the assembly is transported, or thermally or mechanically stressed. Typically, these height variations and misalignments may be compensated for by use of thermal gap fillers or thick layers of thermal grease, both of which have low thermal conductivity (e.g., 1-8 Watts per meter-Kelvin (W/m-K)). However, thermal gap fillers and thermal grease layers add considerable thermal resistance between a component and the ambient air surrounding such component (such ambient air, as will be well understood by those skilled in the art, forming the component's ultimate heat sink). Unfortunately, this results in an undesirably large increase in component temperatures. Moreover, in practice, the thermal grease is squeezed out of the gap as the components are pressed together, and then, if the gap re-opens up it cannot refill the gap. Thus, the thermal resistance of the gap can actually be much higher than if completely filled with thermal grease.
Therefore, it would be desirable to have a low thermal resistance heat dissipation technique to accommodate for the variations and dynamics of individual components mounted in EMI shielded circuit pack arrangements.